1. Field of the Invention
The present invention relates to a furnace wherein the charge of workpiece is carried during heat treatment by a circular conveyor system.
2. Description of the Prior Art
Similar furnaces are known as rotary hearth furnaces consisting of a cylindrical or a rectangular furnace chamber and a rotating hearth, the charge to be treated being placed on the slowly turning hearth and travelling on said moving hearth through the different furnace zones. The hearth is mounted on bearings outside the furnace and rotated by a drive mechanism also located outside the furnace. Rotary hearth furnaces are usually operated in a cycle mode and can only be charged when a piece has been removed from the furnace. The charged piece travels stepwise from the furnace entry through the heating zone and the soaking zone to the furnace outlet.
The use of such rotary hearth furnaces has become a standard practice for heat treatment, but it is impossible to adjust the length of a rotary hearth furnace cycle. A given time is hence always required for a certain charge to travel through a given rotary hearth furnace, the rotary hearth furnace design thus determining the length of the heat treatment cycle.
It is further difficult to implement gas treating processes such as nitriding in rotary hearth furnaces, since the time during which the charge remains in the soaking zone is usually too short. Moreover, each hot tray carrying the heat-treated charge rests in the immediate vicinity of a cold tray entering the furnace, thus making it practically necessary for an empty position to be between the discharging position and the charging position. However, such an empty position is an inadequate design concept for modern heat treating applications. If a rotary hearth furnace is mounted with a charging door and a separate discharge door, radiant tubing in the area of said empty position may provide a certain thermal separation between the furnace charging zone and the furnace discharging zone. It would also be possible to separate the two zones by a baffle if arrangements are made for returning trays without their inserts from the discharging position to the charging position. Further, large rotary hearth furnaces include a large unused area in their center since it is impossible to separate cold atmosphere from hot atmosphere. In order to eliminate some of the disadvantages described, pusher-type furnaces have been used for continuously treating metal in a gaseous atmosphere. However, the surface area required for the installation of such pusher-type furnaces is considerable. Also, the chambers of pusher-type furnaces are large and therefore the energy input for heat-treating the metal pieces is high.
Additionally, trays and tray inserts for pusher-type furnaces account for some 20 to 50% of the gross charge weight. In the case of a pusher-type furnace they must be made of heat-resistant material and will be heated and cooled during each journey through the furnace, thus increasing energy consumption, and subjecting them to early wear.